The invention relates to a support material for photographic and non-photographic imaging processes with a raw paper and at least one thermoplastic layer arranged on the front of the raw paper.
The principle is known of refining papers for photographic and non-photographic imaging processes by surface combination with at least one polyolefin layer by extrusion, co-extrusion, or lamination. The purpose of this is to improve the appearance and properties of the base material. Coating with polyethylene plays a major part in this.
For coating the front side of papers for imaging processes, low-density polyethylene (LDPE) is usually used. The advantages of this polymer lie mainly in the price and in the largely trouble-free processing. Thus, in particular, the extrusion of a film with uniform thickness is manifested without any problem. Photographic papers coated with polyethylene can undergo further processing without any problem; i.e. they can be coated with light sensitive emulsion layers. Very good adhesion between the gelatines of the photoemulsion and the coated paper can be achieved by corona treatment of the PE paper. Polyethylene coatings also feature a range of disadvantages in comparison with coatings with other polymers, however. Under thermal treatment, the LDPE molecules are inclined to cross-linking reactions, which substantially increases the melt viscosity. This mechanism leads to unevenness of the paper base layer which is to be coated, which result from the fibre structure, and which can only be compensated for to an unsatisfactory degree during the extrusion coating. This results in a reduction of gloss and smoothness of the coated paper. In addition, an increase may be observed in the thickness fluctuations in the middle image tones and what is referred to as mottle on the papers coated with photographic emulsions and developed.
Polypropylene does not feature these problems in thermoplastic processing. During processing, a drastic reduction in melt viscosity is observed, which can be explained by partial breaking of the polymer chains without subsequent cross-linking reactions. Polypropylene is accordingly clearly better suited than LDPE to equalize unevenness in the paper base layer which is to be coated, and so leads to an optically smoother surface of the extruded paper.
However, a series of problems arise at the coating of the polypropylene and during further processing. With polypropylene, a uniform coating thickness of the extruded film can only be applied with great difficulty during the extrusion coating, because the inadequate film stability of the film extruded from the nozzle leads, for example, to constrictions and therefore to considerable application fluctuations in the range of several g/m2. Papers which were coated with a polypropylene homopolymer or also with a polypropylene/polyethylene copolymer show, at the application of a photographic emulsion, a drastic deterioration in the adherence of the photographic emulsion to the carrier. This problem cannot be compensated for even by a stronger corona treatment. In addition to this, with the use of polypropylene/polyethylene copolymers, the polypropylene-specific properties are in part lost.
EP 0 880 065 A describes a photo carrier material which is characterised by an especially smooth surface and high rigidity. This carrier material consists of a paper support and a multi-layered film on the front side. The film is composed of at least three layers, and is biaxially oriented. For preference polypropylene films are extruded. Paper support and film are joined by means of an extruded polyethylene layer. Disadvantageous in this context are the high production costs and the inadequate adherence of the photographic emulsion. To improve the adherence, an additional adherence layer must be applied.
EP 0 880 066 A likewise describes a photo carrier material on a paper support and a multilayer polypropylene film. This film is extrusion-coated with a polyethylene. As a result, a readily-attainable adherence of the photographic layer is achieved, with a good surface, but this again increases the costs in relation to a standard photo support material. Disadvantages are again the adherence problems which arise between the polypropylene film and the extruded polyethylene layer.
JP 7 092 607 A describes a photographic carrier material coated with polyethylene with high gloss and good adherence. The single polyethylene layer contains, as its main constituent, a polyethylene manufactured by means of metallocene catalysts. The Mw/Mn ratio with this polyethylene is greater than 5. Disadvantageous with this paper is the inclination to mottle. No increase in rigidity can be achieved by means of this support material.
JP 7 270 969 A describes a photo carrier material which can be coated on the front side of a polyethylene layer and a polypropylene layer. In this situation the polypropylene layer can be applied as the upper layer. This results in a support material with a high gloss. The emulsion adherence with this formulation, however, is inadequate; adherence between the polymer layers is not satisfactory either.
In addition to the surface properties, the strength values of the support material play an important part. With the use of films on materials similar to films, good surface qualities and high rigidity values can be achieved. Properties such as resistance to tearing are in general impaired. With thermoplastic coated papers, with only one resin layer, strength properties such as resistance to tearing are, as a rule, good.
The object of the invention is to provide a support material for photographic and non-photographic imaging processes with a smooth surface, similar to a film, which on the one hand allows for good adherence of the photographic emulsion or the recipient layer, and, on the other, good adherence between the different thermoplastic polymer coatings, with good mottle behaviour, high rigidity and tearing resistance with, at the same time, low production costs.
The problem is solved by a support material comprising a raw paper provided with a thermoplastic layer on the front side, whereby the thermoplastic layer contains at least one polyolefin layer arranged on the raw paper and at least one polyethylene layer, and at least one of these layers contains a polyethylene obtained by means of metallocene catalysts. For preference, the polyethylene layer contains the metallocene polyethylene.
Metallocene polyethylenes are copolymers of the ethene with xcex1-olefins such as butene-1, hexene-1, or octene-1, manufactured with the aid of metallocene catalysts. These polyethylenes and there manufacture are known.
A polyethylene layer is applied onto the surface of a raw paper by means of (co) extrusion of at least one polyolefin layer and at least one polyethylene layer. The polyolefin layer is in this case located on the raw paper. The polyethylene layer is arranged above the polyolefin layer. For the polyolefin layer, a variety of different polyethylene types can be used, such as HDPE, MDPE, or LDPE. These polyolefins can be manufactured by means of Ziegler or also metallocene catalysts. The use of polypropylenes, polybutylenes, and other polyolefins is also possible.
For preference, the lower layer is a polypropylene layer. By means of this layer arrangement, the positive properties of the lower layer, such as uniform coating of the raw paper and high gloss, can be combined with the good adherence properties of the polyethylene.
The application weight of the polyolefin layer can be 10 to 35 g/m2, for preference 15 to 25 g/m2. The application weight of the polyethylene layer is 2 to 30 g/m2, for preference 10 to 20 g/m2. All known polypropylene types are suitable for the polypropylene coating. Propylene homopolymers and/or propylene copolymers can be used. Mixtures of polypropylene with other polyolefins can be used. Especially preferred are polypropylene types with long-chain branches (HMS types). To improve opacity, pigments can be added such as titanium dioxide, clay (kaolin), calcium carbonate, or zinc oxide. Titanium dioxide may be present in the layer in a volume from 0 to 15% by weight, for preference 5 to 12% by weight. Titanium dioxide can also be used in the anatase or rutile form.
For the polyethylene coating, use is made for preference of a mixture of standard LDPE (density 0.9xe2x88x920.936 g/cm3) and an LDPE, manufactured by means of metallocene catalysts. Particularly suitable are metallocene polyethylenes with an Mw/Mn ratio of xe2x89xa63.5. Particularly preferred is an Mw/Mn ratio of 1.8 to 2.8. The proportion of metallocene polyethylene is 10 to 60% by weight, for preference 20 to 40% by weight. Particularly well-suited is a metallocene LDPE with a density from 0.70 to 0.90 g/cm3. To increase the opacity and whiteness, it is likewise possible for the pigments used in the polypropylene layer to be added. For preference, use is made in this layer too of titanium dioxide in a volume of 7 to 20%, and in particular in a volume of 10 to 16% by weight, in one of the two crystal forms referred to heretofore.
The reverse side of the raw paper can be coated in the same manner as the front side. For preference, use is made of an extrusion coating with a polyethylene homopolymer and/or copolymer. The application weight of the back side layer is 5 to 60 g/m2, for preference 10 to 40 g/m2.
All types of cellulose fibres and synthetic fibres are suitable for manufacturing the base paper. Coniferous and/or deciduous pulps can be used, macerated by alkaline or acidic processes. For sizing, all known sizing agents and wet strength agents known in the paper industry are suitable. All inorganic compounds such as clay (kaolin), calcium carbonate, and titanium dioxide are suitable as pigments. The base paper may contain additional additives and auxiliary agents such as defoaming agents, optical brighteners, and colouring agents. It can be manufactured on Fourdrinier or Yankee paper machines. The basis weight of the base paper may amount to 40 to 300 g/m2. To improve the surface properties, the base paper can be treated with a smoothing device or a calander.
The coating of the raw paper according to the invention with the thermoplastic polymers can be effected by extrusion or coextrusion. In this situation, the polymer layers can be applied one after another or simultaneously. The extrusion speed may be between 100 and 500 m/min.
The base paper can be provided with further function layers such as an anti-static layer or printable layers on the front and/or reverse.